1. Field of the Invention
The present invention relates to improvements in a handpiece having a root canal length measurement function for a dental treatment apparatus.
2. Description of the Prior Art
An apparatus capable of conducting root canal treatment while performing root canal measurement by using a cutting tool, such as a file or a reamer, which is also used as a measurement probe, is known. In this case, since the cutting tool is used as one of the electrodes of a root canal length measurement circuit, measurement signals are required to be sent to the cutting tool via lead wires. Such lead wires are conventionally connected by way of the outside of the handpiece of an apparatus. This causes the problem of impairing the operability of the handpiece because of nuisance due to the existence of the lead wires, and also causes the problem of requiring the troublesome task of externally connecting the lead wires to the head of the handpiece at each treatment time.
In order to solve these problems, the inventors of the present application paid attention to the fact that the drive power transmission mechanism in the handpiece comprises metallic members, namely electrically conductive members, and have proposed that a tool holding mechanism should be connected to the root canal length measurement circuit by using the drive power transmission mechanism as a signal circuit (refer to Japanese Laid-open Patent Application 7-328034, for example). However, in this proposal, the signal circuit may be opened instantaneously as gears are rotated, raising the possibility of unstable measurement operation. Furthermore, in a type of cutting tool, the cutting tool's shank portion installed in the measurement probe holding mechanism of the handpiece and the cutting portion at the tip of the cutting tool are insulated from each other. This type of cutting tool cannot be used for the handpiece proposed above.